1. Field of the Invention:
This invention relates to a nuclear fuel element having a tag gas sealed therein for minimizing the accident such as breakage of a cladding tube.
2. Description of the Prior Art:
A number of fuel assemblies are arranged in a nuclear reactor. The fuel assembly used in a BWR consists of a number of fuel elements (rods) arranged in a lattice form within a channel. In a first breeder reactor, the fuel assembly consists of a number of fuel elements (pins) arranged in a honeycomb form within a wrapper tube. The fuel element is prepared by loading in a laminate form a number of pellets within a cladding tube and has plenum in the upper portion. The pellet is prepared by compression molding, followed by sintering, of a fuel material such as uranium dioxide. The fuel element thus prepared is subjected to fission reaction within a reactor and the tremendous heat generated by the fission reaction is taken out by a coolant.
When the cladding tube has been broken undesirably during the reactor operation, the fission product leaks within the reactor, resulting in contamination in and around the reactor as well as in trouble in the reactor operation. In order to detect breakage of the cladding tube in an early stage for preventing a serious accident, it is customary to seal a tag gas of particular composition within a cladding tube. If the cladding tube within the reactor has been broken, the leaked tag gas is detected, rendering it possible to know from the composition of the leaked tag gas which of the cladding tubes has been broken. Such a method of detecting an accident is called a gas tagging and the fuel element used is called a tag gas-sealed fuel element.
A conventional tag gas-sealed fuel element is of the type that a particular capsule having a tag gas-sealed therein is loaded in a cladding tube. The tag gas is released by some means from within the capsule into the cladding tube. For example, a conventional element is disclosed by C. A. Strand and R. E. Schenter in "Nuclear Technology", Vol. 26, pp. 472-479, August 1979. In this case, a tag gas-sealed stainless steel capsule having a thin film portion is loaded in the plenum of a cladding tube and a fuel element is assembled by sealing the both ends of the cladding tube. The thin film portion of the capsule is broken by a magnetically moving penetrator so as to release the tag gas within the cladding tube. Also known is a method in which the thin film portion of the capsule is broken by a penetrator which is moved by thermal expansion in accordance with temperature elevation of the reactor. Japanese Patent Publication No. 22635/78 discloses an additional technique. In this case, a capsule containing a tag gas is sealed by an alloy having a melting point lower than the operation temperature of the reactor so as to permit the alloy to be melted during the reactor operation for releasing the tag gas within the cladding tube. In general, the tag gas leaked within the reactor vessel by breakage of the cladding tube is detected by a mass spectrometer as shown in FIG. 1 of "Nuclear Technology", Vol. 26, p. 473 mentioned above.
The conventional tag gas-sealed fuel element described above leaves room for further improvements. Specifically, where tag gas-sealed capsule having a thin film portion is loaded in a cladding tube, the fuel element must be assembled very carefully so as not to break the thin film portion, leading to a very inefficient assembly work. Further, the assembled fuel element must be handled very carefully for preventing the thin film from being broken undesirably. Still further, it is extremely difficult and troublesome to seal a tag gas-containing capsule with a thin film. The technique utilizing a penetrator which is moved by thermal expansion for breaking the thin film gives rise to difficulty in achieving construction of the assembly which permits imparting the penetrator with a sufficient force for breaking the thin film, resulting in a low reliability. The technique utilizing a sealing alloy having a low melting point is also unsatisfactory in reliability. Specifically, the sealing alloy tends to peel off or drop because of the impulse or vibration in the constructing step of the fuel element unless the alloy is firmly bonded to the capsule. Further, when it is required to release the tag gas within the cladding tube before operation of the nuclear reactor, many portions of the fuel element must be heated from outside, resulting in that bad influences are given to the fuel element material. In addition, the alloy having a low melting point fails to be melted in some cases during operation of the nuclear reactor because of insufficient heat conduction.